First, let me say, not all my vamps are daywalkers. In fact, most are not. Being able to walk in sunlight is a trait that’s either inherited or learned. Learning is a very strenuous process that can take centuries. It’s very painful. So most younger vampires won’t take time to learn it. If they weren’t born with it, they’ll just move on.
I treat my vampires like I treat my humans in that they are a species and as such are able to evolve. Of course this takes time. But you see throughout the course of the story, other abilities that vampires have developed everything from acid spit to self-cloning. The ability to walk in daylight then becomes merely another trick that some have adopted.
And actually all of them, at some point, had to be learned by someone. So even with daywalking, that skill had to be learned and passed on.
Daywalking however is one of those traits that can make a vampire incredibly dangerous. In the beginning, it was very carefully guarded and passed down to only certain people, the “chosen ones” if you will. Then the wrong person got a hold of it, passed it to the wrong people, and here we are. A whole family of daywalking nutjobs passing the power on others of their kin.